The intense, but unwanted, electromagnetic or acoustic noise originating from a distant source could be disruptive to communications or could be an intolerable nuisance. The acoustic noise from a moving vehicle, such as large commercial aircrafts and helicopters, for example, can far exceed the reasonable, tolerable limit of noise, particularly at regions close to many airports. Often sources of such noise are unavoidable and means available to combat them are limited, if any. In the case of the noise due to an aircraft, the severity of the problem has compelled regulatory agencies in many areas to set upper bounds of maximum tolerable noise level permitted by an aircraft, and to enforce procedural means to reduce noise in communities close to major airports. This nuisance problem has led to an expansive research and development effort to design and build jet aircraft engines that produce less noise. Although a significant improvement in the aircraft noise reduction has resulted from this effort, the noise level still remains unacceptable and the corresponding nuisance remains unabated in many areas of the world. Since the aircraft noise reduction by better design of engines, exhaust fans, etc., is seemingly approaching a technological limit such that additional noise reduction is only feasible at the expense of performance degradation, complimentary technology and devices that can aid the noise abatement problem, are being sought by aircraft manufacturers, airlines, regulatory agencies and community managers, alike. One objective of the invention is to offer a remedy for further noise abatement in a specific region or regions of concern, beyond what is achievable by improved aircraft design.
A similar problem also arises for the electromagnetic noise where the existence of such noise can not be avoided, and conventional means available at the victim region to reduce the effect of interference, are either impractical or impossible to implement.
In many cases, the abatement of the noise becomes further involved because the noise source is a moving platform and the distance, and direction of the noise source with respect to the victim region are not precisely known to use a remedy which is not adaptive in character. If, however, a counter-noise can be created such that it always accompanies the noise of concern at the victim region, and if the noise and counter-noise at the victim region are equal in amplitude and 180.degree. out-of-phase, then the ill-effects of the noise can be significantly reduced, if not eliminated. Since exact characteristics of the noise, such as its amplitude, phase and spectrum are always not a-priori known, an adaptive system is necessary to set and maintain the required counter-noise, amplitude, phase and spectrum such that the sum of the noise and counter-noise at the victim region of concern approaches a minimum. A cooperative sensor at the victim region, that can continuously monitor the sum of the noise and counter-noise at the victim region, and can provide means to control the amplitude and phase of the counter-noise so that the sum of the noise and counter-noise at the victim region approaches a minimum, could constitute a key element necessary for a closed-loop control of the counter-noise source that assures the noise reduction in a specific region, automatically.
In addition to an equal amplitude and 180.degree. out-of-phase relationship between the noise and counter-noise, one needs to ensure that the waveforms of the noise and counter-noise are also identical at all times. This requirement, however, is readily achieved when the counter-noise is produced from a sample of the noise itself, so that its waveform is the same as that of the noise, except for a difference in amplitude and phase. Thus the key elements necessary to reduce the intensity of noise of acoustic or electromagnetic origin become a counter-noise source, a sensor and means to effect a change in amplitude and phase of the counter-noise, depending on the sensor output. The invention described herein utilizes these key elements in an integrated close-loop control system.
With reference to the prior art, an adaptive noise or interference cancellation system for a particular receiver, invented by Ghose and Sauter, has been in existence since 1969. Unlike the present invention, the invention of Ghose and Sauter relates to a radio communication system and is useful for minimizing or eliminating an interference in a particular radio receiver or a system located at a point. The invention of Ghose and Sauter, therefore, can not cure the problem of noise at any other point or receiver, even in close proximity of the receiver for which the noise minimization is intended. The fundamental difference between the invention of Ghose and Sauter and the present one is that in the present invention, the noise source is effectively neutralized by a counter-noise source in-so-far as a broad victim region, instead of a point or a particular receiver, is concerned. An element of the Ghose and Sauter invention, namely a pair of signal controllers responsive to the control signal for varying the amplitude and phase of the noise passing therethrough is however, utilized in the present invention.
Perhaps, in a broad sense, a phased array, where the amplitude and phase of different elements of the array can be varied to create a null in a specific direction, can be regarded as a relevant prior art. The fundamental difference between this prior art and the present invention is that, in the present invention a close-loop control, guided by an "in fact" noise minimum criterion at the intended victim region, is used to vary the amplitude and phase of the counter-noise, which, of course, is not present in a phased-array. The concept of counter-noise source is also absent in a phased array.
Other differences between relevant prior arts and the present invention will be apparent from the specifications, drawings, and claims that follow.